And now we are talking about fuel economy in one of these exotic sports cars..
but yea, if the weight goes down, even the current spec of the V10 will do the job, but if the weight goes down, i would guess they probably need to reduce the weight of the V10 as well, i doubt the engine can be pushed back any further to achieve their 50:50 distribution. And to get 550horses, they dont' even need a 5L engine to do it with turbos and other new techs right?
Wonder if there will ever be diesel M models, REAL efficient dynamics... let's do 8L/100km fuel consumption with 500hp and 800Nm!

But it is too early, it won't hit showrooms until sth like early 2012.